The present invention relates to cervical plates used for fusing cervical vertebrae in the treatment of spinal disorders, and more particularly to components for fastening such plates.
An increasingly accepted procedure for treating spinal disorders involves using substantially rigid plates to hold vertebrae in desired spatial relationships and orientations relative to each other. The upper cervical spine can be approached anteriorly or posteriorly, although anterior approaches are of more interest in connection with this invention. In either case, holes are drilled and tapped in at least two of the vertebrae, to receive screws or other fasteners used to secure the plate. The holes are accurately positioned with reference to openings formed through the cervical plate. In some cases the screws may be self-tapping. Typically the plate is curved about its longitudinal axis to facilitate contiguous surface engagement of the plates with the vertebrae. With the plate maintained against the vertebrae, the fasteners are secure within the holes. As a result, the plate maintains the attached vertebrae in a desired spacing and orientation with respect to each other.
One of the problems associated with this technique is the tendency of screws or other fasteners to gradually work loose after fixation. Slight shock or vibration of the vertebrae, due to walking, climbing stairs or more vigorous activity by the patient following treatment increases this tendency, jeopardizing the integrity of fixation. Moreover, as the fasteners work loose, the outward protrusion of the heads over other components of the fasteners can be a source of discomfort and present the risk of trauma to adjacent and surrounding soft tissue.
The curvature of cervical plates typically results in a convergence of fasteners that extend through spaced apart openings in the plate, particularly when each screw is perpendicular to the region of the plate surrounding it. Screws sufficiently short to avoid interfering with one another may not be long enough to assure a secure plate fixation. Further, the physician may encounter difficulties in positioning the plate if one of the vertebrae, due to a particular shape and orientation, cannot readily retain a perpendicularly inserted fastener.
There have been attempts to solve the above problems. For example, screws or other fasteners can be provided with somewhat rounded heads as shown in U.S. Pat. No. 5,324,290 (Zdeblick). U.S. Pat. No. 5,261,910 (Warden, et al.) shows a nut with a rounded upper surface and a hexagonal recess. The nut has a conical portion that fits into a similarly shaped recess in a plate, thus to reduce the height at which the nut extends above the plate. U.S. Pat. No. 5,364,399 (Lowery, et al.) discloses an anterior cervical plate system in which the openings through the plate guide the screws in a non-perpendicular orientation that causes the screws to diverge, rather than converge, as they proceed into the particular cervical vertebra. The plate has a recess for limiting the degree of outward protrusion of the screw heads. After each pair of screws is fully inserted, a locking screw is threaded into the plate until its head encounters the heads of the bone screws.
While the above approaches yield favorable results in certain circumstances, there remains a need for greater flexibility in positioning and orienting the bone screws or fasteners, and for a simpler, more reliable means of counteracting the tendency of the bone screws to work loose after cervical plate fixation.
Therefore, it is an object of the present invention to provide a cervical plate and fixation system in which bone screws or other fasteners are more securely retained and less likely to work loose, without the need for auxiliary screws or other additional fixtures.
Another object is to provide a fastening system in which the heads of the fasteners are recessed within the cervical plate to minimize their protrusion beyond the plate.
A further object is to provide fastener receiving apertures in cervical plates that are shaped to allow a variety of angular orientations of the fasteners with respect to the plate.
Yet another object is to provide a system including cervical plates with locking features for retaining fastener heads, in combination with tools for conveniently manipulating the locking features to selectively retain or release the fasteners.
To achieve these and other objects, there is provided an appliance attachable to osseous or bony material within a body. The appliance includes a biocompatible structural member having an exterior surface including opposite first and second exterior surface regions. An interior surface region between the exterior surface regions defines an aperture through the structural member. A biocompatible fastener is provided, including an elongate longitudinal shank and a head larger in diameter than the shank. The shank is adapted for an insertion through the aperture and a penetration into osseous material to a depth sufficient to bring the first exterior surface region into a surface engagement with the osseous material while the head engages the interior surface region. Thus, the head tends to maintain the surface engagement to fix the structural member substantially integrally to the osseous material. A retaining element is permanently fixed to the structural member. The retaining element is moveable from an open position for allowing travel of the head into the aperture and against the interior surface region, to a closed position for maintaining the head against the interior surface region.
Preferably the retaining element is resilient and ductile, and moveable from the open position to the closed position by applying an external force above the elastic limit of the retaining element. The retaining element can be a tab supported in cantilevered fashion, or alternatively an elongate member attached at two opposite ends to the structural member. As another alternative, the retaining element can be elastic, normally (when not subject to external stresses) tending to assume to the closed position, and elastically deformable into the open position when subject to an external force.
The fastener head, when maintained in the aperture against the interior surface region, is advantageously contained between the first and second exterior surface regions, so that the head does not protrude outwardly beyond the plate or other structural member.
According to another aspect of the invention, the aperture is sized with respect to the shank to allow a pivoting of the fastener with respect to the structural member about at least one transverse axis. More preferably, the fastener is pivotable about all transverse axes passing through a given point in the first aperture, thereby defining a conical volume within which the fastener is selectively positionable.
The appliance preferably includes a second fastener substantially identical to the first fastener, and a second aperture through the structural member for receiving a head of the second fastener when a shank thereof is inserted through the second aperture. Then, for example, when the osseous material comprises two vertebrae, the structural member is positioned for extension of the first and second fasteners through their respective apertures into different ones of the vertebrae. Thus the fasteners cooperate with the structural member to support the vertebrae substantially integrally with respect to one another.
A system including the structural member and fasteners can further include tools for securing and removing the structural member, particularly in conjunction with resilient and ductile retaining elements. In particular, heads of the fasteners can include non-circular recesses, and a drive tool with a drive shaft can be provided, one end of the shaft having a non-circular profile corresponding to the profile of the recess. The fastener shanks, in this approach, are externally threaded, with a drive tool rotatable to turn the fasteners. The drive tool further can incorporate a sleeve coaxial with the drive shaft and incorporating flexure members to grip the fastener being turned by the drive tool.
Preferably, locking and releasing tools also are provided. The locking tool can include a shaft with a locking end positionable against the retaining element and movable to plastically deform the retaining element, moving the element into the closed position. The releasing tool has a releasing end positionable against the retaining element when the same is in the closed position. The releasing end is movable to force the retaining element into the open position to allow a withdrawal of the associated fastener.
Thus in accordance with the present invention, cervical plates and other structural members can be secured to vertebrae or other osseous material in a manner that more reliably prevents fasteners from working loose in response to shock or vibration. Resilient tabs or retaining members are moveable, through either plastic or elastic deformation, to open positions that allow insertion and removal of fasteners, and alternatively are positioned to prevent fasteners from working free of their respective apertures in the cervical plate or other structural members. The ability to select non-perpendicular angles at which the fasteners extend from the cervical plate permits relatively close positioning of fastener apertures without the risk of the fasteners interfering with one another, and more generally allows each fastener to be aligned for its most convenient or most secure angle of penetration into a vertebrae or other bony material.